Indicating means for inclosed fuses.



D. C. HOOKER.

I'NDIGATING MEANS FOR INOLOSED FUSES.

APPLIOATIO! FILED FEB. 7, 1918.

1 ,O87,12 O, 1 Patented Feb. 17, 1914.

nouns rmns. M Luna, Hum-diam n c.

UNITED STATES PATENT OFFICE.

DUNCAN C. HOOKER, F FARMINGTON, CONNECTICUT, ASSIGNOB TO THE JOHNS-PRATT COMPANY, OF HARTFORD, CONNECTICUT, A CORPORATION OF CONNECTICUT.

INDICATING MEANS FOR INCLOSED FUSES.

Specification of Letters Patent.

Patented Feb. 1'7, 1914.

Application filed February 7, 1913.. Serial No. 746,808.

indicating means for inclosed electric fuses of small capacity.

Hitherto it has been considered impracticable to provide an inclosed fuse of small capacity, for instance two amperes and below, with means for indicating Whether or not the fuse is good or blown. The fusible element of such a small fuse is necessarily so slight that its mechanical strength is not sufficient to restrain a spring or tension device, such as is employed with any of the mechanical indicators at present in use; the disruption of such a small fusible'element is accompanied by so little violence that it is insuflicient to operate any indicator depending upon pressure; andthe re sistance of such a fine fusible element is too high and its conductivity too low to permit the use of a shunt wire indicator, similar to that commonly applied to inclosed fuses of greater capacities. When two conductors are in parallel, the amount of current carried by each will be inversely proportionate to their resistances. It is evident therefore that if the main fusible element is made small enough to constitute a properly calibrated fuse of two amperes capacity or below, the conductivity of the indicator wire will be so large in proportion that its share of the total current carried by the two will be suihcient to cause-it to heat to a point where the indicator spot will ignite or discolor, even though the main fusible element is not blown. Another reason for the impracticability of a shuntcircuit indicator for such a small fuse is that the indicator wire itself is capable of carrying so much, current that no matter how small the conductors, the device would carry more current than its rating permits.

The object of this invention is to overcome the dilliculties mentioned and provide a practical indicator for a fuse of small ca-' pacity.

Referring to the accompanying drawings, Figure 1 shows a side view of an mclosed fuse provided with an indicator which embodies this invention, the indicating label being omitted from this view. Fig. 2 shows a longitudinal section of the same taken through the indicator and label.

A common type of inclosed fuse is illustrated. This fuse has a tubular inclosing case 1 of fiber or other suitable inshlating material. The ends of' this tubular casing are usually closed by brass terminal caps 2.

Extending inwardly and in contact with these conducting end caps are the internal terminals 3 ordinarily formed of copper wire. These interior terminals are joined by the fusible wire or stri 4. The casing may or may not be filled with the commonly employed granular non-conducting materia The indicating wires 5 may have their ends connected with the conducting terminals in any desired manner. The drawings show the ends of these wires as electrically connected with the end caps bymeans of metallic eyelets 6. In the construction illustrated the indicator wires extend inside of the casing for a distance from the 'end caps and near the middle pass through holes to the exterior. The outer ends of" these indicator wires are not electrically connected with each other. The end of one is shown as bent down against the outer surface of the tubular caslng and the end of the other is similarly bent'insuch manner that the two ends cross or overlap, without however, making electrical contact with each other. An insulating sheet 7, of paper or other suitable non-conducting or high resistance material is inserted between the ends of the two wires and then over this may be placed a label 8 provided on the inside with a spot 9 of fulminating paste or other readily inflammable material.- Vvhen a fuse constructed in this manner is placed in an electrical circuit so as to form a part thereof, all the current will pass through't-he fusible element-the wire or strip L-and none through the indicator wires,.as the ends of the indicator wires arev separated by material having too great resistance. The indicator wires, however, are alive as their ends are connected with the conducting terminals. As long as the fusible elementremains in tact the insulating sheet of paper or other substance between the ends of the indicator wires will be subjected to no electrical stress excess current flow, or in other words when the fuse is blown, the insulating sheet tween the ends 01' the indicator wires is sub gected to the full potential of the circuit in which the fuse isinstalled, as one of the terniinals will be alive and the other dead. The stress of this line voltage is suflicient to break down or puncture the insulation or overcome the resistance and thereby reflect an electrical connection between the ends of the indicator wires. These Wires will then rapidly heat and burn off causing the indicator spot to ignite or discolor and show a black spot on the label which wiliindicate that the fuse has been. blown. The electrical connection formed between the overlapped ends of the indicator wires when the main fuse is blown is accompanied by a very slight arcing at the point of their intersection and the resulting heat can be utilized to indicate the blowing of the fuse in any of the well known Ways employed for this purpose with the larger sizes of-fuses. The sheet of paper which is desirably used has sufiicient dielec trio strength to resist a drop of potential 5 caused by the resistance of the fusible ele ment but is insufiicient to resist the potential of the line. after the fusible element is dis rupted, or resist the surge or kick of the current at the time the fuse opens. tori-a l than thin paper may be employedvfor this purpose if desired, such for instance as a sheet or piece of-material that would coin- 'IHOIllY be classed as a conductor but which has a resistance so high that it would prevent any appreciablefiow of current when subject-ed only to the potential due to the Other Inarooms .0

drop across the fusible element, yet would permit enough current to flow when sub" jected to the full potential of the line to cause the operation of the indicator.

The invention is not limited to any specific fori'n of indication which might be brought about by the operation of the indicator wires, or to the particular arrangement of insulating sheet 'which sepa ates the free ends of the indicating wires, or to the spccitic construction. of the casing or conducting terminals illustrated in the drawings.

The invei'ztion claimed is:

1. An inclosed fuse having a fusible conductor, a sectional indicating conductor with the ends of the sections crossed on the exte *ior of the fuse, and a sheet of paper of dielectric strength su'liicient to resist a drop of potential. caused by the resistance ot the fusible element but insufiicient to resist the potential of the circuit after the fuse is dis ruptect, inserted between the crossed. ends of the sections of the indicating conductor.

2. An incloscd fuse having a fusible conductor, a sectional indicating conductor with the ends of the sections crossed on the ox terior the fuse, a sheet oi. paper of dielectrio strength suiiicient to resist a drop of potential caused by the resistance of the fusible element, but insuliiciont to resist the potential of the circuit after the fuse is disruptech inserted between the crossed ends 01'' the sections of the indicating conductor, fuln'iinate applied t o the exterior of the fuse adjacent to the crossed ends of the indicating conductor, and a label coveringkthe iulininatc.

I DUNCAN C. l l (.)OKER.

Vvitnesses lll itRRY R. YVn'iLmMs,

JOSEPHINE M. Srnmrrrnn. 

